Chinese Patent No. 200420082546.8 describes a crystal growing device that can effectively grow a single crystal using a dual heating temperature gradient method. Similar to the traditional temperature gradient method, during the crystal growing process a heater is located at partial height of a crucible, and gradual crystal growth is achieved through moving the heater at a proper speed. This type of method is effective in growing a single crystal, but cannot be used to realize the growth of multiple crystals simultaneously.
In prior art multiple crucible crystal growing technologies, a crystal growing system generally employs a Bridgman method. This type of system elongates the hearth of the furnace in one direction, so that the hearth can house multiple growing crystals. The near rectangular temperature field formed in the elongated direction under the condition has achieved initial success of growing lead tungstate crystal that has a square outer appearance. In this type of Bridgman crystal growing system, the high temperature zone in the hearth is usually the whole chamber area above the crystal growing point, and the crystal growing material and doping agent are in molten state. When growing a crystal with a doping agent such as La2O3 that has high melting point and low volatility, the space above the solid part that is preformed or formed due to volume reduction from growing polycrystalline raw materials to single crystal product has no apparent effect on crystal growth. On the other hand, when growing with a doping agent that has low melting point and high volatility, such as PbF2, the doping agent can severely evaporate into the space above, causing doping almost impossible to realize. In prior art multiple crucible Bridgman crystal technologies, the crucible supporting device in a crystal growing system, such as that grows lead tungstate crystal, Sb2O3 doped lead tungstate crystal, or Nb2O5 doped lead tungstate crystal, is controlled by automated equipment and can cause crucibles to move downward to realize crystal growing according to requirements. However, this type of crystal growing crucible supporting device can only cause the crystal growing crucibles to move up and down. During the crystal growing process the melt and the crystal are at rest with each other, only natural convection and dissipation occur in the melt. The primary driving force for crystal growing, therefore, is particle concentration gradient, temperature gradient in the melt and gravity. Because natural convection and dissipation are the only vehicles to move the growing particles, particle transfer rate in the melt is slow and efficiency is not high. When growing a crystal that has simple components, similar component ion effective segregation coefficients, crystal composition at the growing interface similar to the melt composition, particle transfer in the melt is not very important. And the prior art Bridgman system can do the job. When growing a crystal that has more complex components, crystal composition at the growing interface relatively different from the melt composition, and clearly different component ion effective segregation coefficients, insufficient particle transfer ability in the melt causes the particles that are needed for crystal growing not able to get to the interface and those particles that are not needed at the interface, for example, impurity, not able to transfer into the melt instead to accumulate in at the interface. When concentration of the unwanted particles gets too high, impurity or even a secondary phase can form in the crystal. It can cause the crystal to have too many defects, even severe defects such as secondary phase inclusion. Or, it may cause severe consequences such as inconsistent doping agent ion distribution in the doped crystal—unacceptable for crystal quality.
In a multiple crucible Bridgman system the near rectangular hearth can hold a few and even up to tens of growing crystals simultaneously, realizing the purpose of multiple crucible crystal growing. The heater located at the hearth wall can satisfy the heating requirement of crystal growing. The near rectangular and asymmetrical temperature field in the length direction is suitable for growing crystal that has a square outer profile. The whole growing process is very quiet. And the system is beneficial to growing crystals that have low requirements for particle transfer in the melt, and with components or doping ions having similar effective segregation coefficients. It is not beneficial to growing round crystals. The system is characterized by the melt and the crystal at rest with each other during the whole growing process, lack of forced convection, and low solvent transfer rate. It is not beneficial for growing crystals that have big difference between component ion effective segregation coefficients.